There are high expectations for the development of carbon nanotubes (hereinafter, also referred to as “CNTs”) in functional new materials such as in electronic device materials, optical element materials, conductive materials, and bio-related materials. To this end, carbon nanotubes have been studied from various angles, including use, quality, and mass producibility.
One known CNT producing method is a chemical vapor deposition method (hereinafter, also referred to as “synthesis method”; see Non-Patent Document 1, Patent Documents 1 and 2). The method is characterized by contacting a feedstock gas such as a carbon compound with particles of catalyst in a high temperature atmosphere of about 500° C. to 1,000° C., and enables CNT production under varying conditions, including the type and placement of a catalyst, the type of a feedstock gas, reducing gas, carrier gas, synthesis furnace, and reaction conditions. Because of these characteristics, the method has attracted interest as being suited for CNT mass production.
The synthesis method also has other advantages, including the ability to produce both single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), and the ability to produce large numbers of CNTs vertically aligned on a base surface supporting a catalyst.
The single-walled CNTs, in particular, have attracted interest as material of, for example, electronic devices, capacitor electrodes, MEMS members, and fillers of functional materials for their excellent characteristics, including electrical characteristics (extremely high current density), thermal characteristics (heat conductivity comparable to that of diamond), optical characteristics (emission in optical communication band wavelength ranges), hydrogen storage capacity, and metal catalyst supporting ability, and for their characteristics as both a semiconductor and a metal.
With the conventional chemical vapor deposition method, however, carbon impurities that generate during the synthesis of CNT cover the catalyst particles and easily deactivate the catalyst, preventing efficient CNT growth. The catalytic activity is typically about several percent with a lifetime of about 1 min. It is therefore common practice in a conventional single-walled CNT growth step to perform synthesis in a low carbon concentration atmosphere.
The low carbon concentration atmosphere as used herein means a growth atmosphere in which the proportion of the feedstock gas in a gas containing the feedstock gas and atmosphere gas is about 0.1 to 1%. Increasing the carbon concentration using a conventional synthesis method deactivates the catalyst even more easily, and further lowers the CNT growth efficiency.
Accordingly, because of the small supply of the feedstock gas to the catalyst, the conventional synthesis method involves a low CNT growth rate, and only enables production of a single-walled CNT assembly as high as about several ten micrometers. The method is also problematic in terms of cost, because the proportion actually converted into CNT from the introduced feedstock gas in the growth step is poor, and the most of the feedstock gas is wasted.
As previously reported by the present inventors (Non-Patent Document 1), it was found that a very trace amount of water or other catalyst-activating substances contained in a reaction atmosphere dramatically improved catalyst efficiency, and enabled production of a high-purity, high-specific-surface-area, single-walled CNT assembly with improved efficiency.
In this method, the catalyst-activating substance added to a CNT synthesis atmosphere removes the carbon impurities covering the catalyst particles, and cleans a base surface of a catalyst layer. As a result, the catalytic activity significantly improves, and the lifetime becomes longer. The improved catalytic activity and the longer lifetime attained by the addition of the catalyst-activating substance increased a single-walled CNT growth time to about several ten minutes from about mere 2 minutes in the conventional method, and improved the catalytic activity to 84% from mere several percent in the conventional method.
This made it possible to obtain a single-walled CNT assembly of a height several hundred times higher than a conventional 4 μm (a height of 2.5 mm in Non-Patent Document 1, a 625-fold increase from 4 μm). This is because the catalytic activity greatly increases in the presence of the catalyst-activating substance, and makes it possible to prevent the catalyst from losing activity even under a high-carbon-concentration environment. The CNT is therefore able to grow for extended time periods at a significantly increased growth rate. As used herein, the “high-carbon-concentration environment” means a growth atmosphere in which the proportion of the feedstock gas in a feedstock-containing gas containing the feedstock gas, atmosphere gas, and catalyst-activating substance is about 2% to 20%.